The northeast monsoon gets into an active phase with high probability of 2019 ending as a normal monsoon year

Heavy rainfall for four to five days since November 28 killed at least 25 people in Tamil Nadu. A wall collapse due to heavy rainfall buried alive 17 people in a village near Coimbatore. Educational institutions were shut down, while some affected people were accommodated in relief camps in Thoothukudi, Cuddalore and Tirunelveli districts, which faced the maximum fury of rains in the state.

"Between November 28 and December 1, Tamil Nadu received heavy rains, thus bringing down the massive rainfall deficit in the state. This bolsters the fact that extremes are the new normal," Sridhar Balasubramanian, associate professor with the department of mechanical engineering, Indian Institute of Technology (IIT) Bombay told Rural Connection. He is also an adjunct faculty member at IDP Climate Studies, IIT Bombay. "There is a high probability of 2019 ending as a normal northeast monsoon year," he added.

Till November 28, the state had minus 43 per cent rainfall departure, which comes in the category of 'deficient' rainfall. This was followed by a heavy downpour for the next three to four days, leading to flooding and loss of lives.

Within four days, by December 2, the state had moved out of deficient rainfall category and registered a rainfall departure of 11 per cent (or, 11 per cent more than normal rainfall). As of December 8, the state has 8 per cent rainfall departure in the northeast monsoon.

As against the southwest season, which extends from June to September and brings rainfall across the country, northeast monsoon (also known as winter monsoon) season is from October to December. Latter brings rainfall over the southern peninsula including five meteorological subdivisions — coastal Andhra, Rayalaseema, Tamil Nadu and Puducherry, south interior Karnataka and Kerala. Tamil Nadu and Puducherry receive the major share of their annual rainfall during the northeast monsoon.

The India Meteorological Department (IMD) data shows the recent daily large excess rainfall received in Tamil Nadu. For instance, on December 2, Tamil Nadu received 'a large excess' rainfall of 345 per cent more than its normal. The following day, on December 3, it again received a 'large excess' rainfall of 186 per cent.

As per the rainfall data of Regional Meteorological Centre Chennai, between October 1 and December 3, the districts of Ramanathapuram, Tirunelveli, Pudukottai, Toothukudi and Coimbatore registered rainfall departure of 73 per cent, 54 per cent, 52 per cent, 44 per cent and 43 per cent, respectively.

"We haven't seen continuous moderate rainfall in the state, as it is supposed to be. Rather it is all spikes and drops. These extremes are a result of climate change, driven by global warming, which is further driven by anthropogenic activities," said Balasubramanian (see graph: Daily mean rainfall for Tamil Nadu/Puducherry in NE monsoon 2019).

Graph: Daily mean rainfall for Tamil Nadu/Puducherry in NE monsoon 2019

According to S Parthasarathy, a Chennai based private weather blogger, Tamil Nadu receives 44 centimetres (cm) rainfall in the three months of northeast monsoon.

"As of December 3, the state has already received 41 cm rainfall, and there may be two more events of rainfall till this month-end. Hence, the state is expected to receive 5-10 per cent more than its normal rainfall," said Parthasarathy.

This is a good news, as last year, there was a huge shortfall in rainfall during the northeast monsoon (minus 24 per cent in Tamil Nadu), triggering acute drought in South India.

"But, Chennai, which still has deficient rainfall [minus 7 per cent rainfall departure, as of December 3], may end up with a rainfall deficit this year, as not much weather activity is expected in northern parts of the state," he added.

Historically, November is the rainiest month for Tamil Nadu. However, this year was quite different.

Balasubramanian explains the reasons: "Two-third of November was very dull, without any significant rainfall activity. This was primarily due to the two cyclones formed in the Arabian Sea —Kyarr and Maha — which took away a lot of moisture from the Bay of Bengal and didn't allow any significant weather system to form in the Bay. These systems are supposed to bring good rains to the state during November. The lone cyclone Bulbul that formed in the Bay also ended up going north due to the ridge action."

Apart from this, the cyclone Maha, unexpectedly, kept looping in the Arabian Sea for a very long time, thereby not allowing the Madden-Julian Oscillation, commonly known as MJO, to propagate into the Bay of Bengal basin. MJO is an eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average.

This delay in MJO moving into the Bay resulted in a subdued northeast monsoon rainfall for the month of November.

Also, it is a known fact that a cyclone takes away a major chunk of thermal and kinetic energy from the ocean and the atmosphere, thereby weakening the ocean-atmospheric exchange, which causes weak monsoon conditions.

This had happened earlier this year in June, too, when cyclone Vayu was formed in the Arabian Sea leading to a delay in the arrival of southwest monsoon in Mumbai. "This time, between October end and November beginning, there were two cyclones [Kyarr and Maha] because of which the northeast monsoon went into a break phase," explained Balasubramanian.

But, the monsoon was sure to return to an active phase, which happened between November 28 and December 2 leading to heavy rainfall in Tamil Nadu.

'Tropical waves'

The main reason behind very heavy rainfall in the state in the last four to five days is 'tropical wave'.

"Tamil Nadu received heavy rainfall because of Rossby Wave, which is a westward-moving tropical wave that moves from Maritime Continent to the Bay of Bengal," said Parthasarathy. "Coupled with this, the MJO was stalled in the Arabian Sea because of which there was lower level convergence over Tamil Nadu leading to very heavy rainfall," he added.

Simply put, tropical wave is a type of atmospheric trough (an elongated area of relatively low pressure), which moves from east to west across the tropics, and results in updraft and cloud formation in the Bay of Bengal. These cloud bands get pushed inland due to the easterly winds and have the capability to dump heavy rains, which they did recently in Tamil Nadu.

This heavy downpour in the state was not a surprise event. "The tropical waves are a very common feature and can be spotted using satellite imagery. Depending on the strength of these waves, they could lead to intense convection and heavy rains. The tropical wave was very strong between Nov 28 and Dec 2, thus resulting in the heavy downpour," said Balasubramanian.

Parthasarathy informed the Rossby Wave has now moved away from Tamil Nadu, and both the tropical wave and the MJO are in the Arabian Sea. "Twenty-seven days of December month are still left and there may be two more incidents [three to four days each] of rainfall in the state. But, these would primarily be limited to southern parts of the state. Overall, the state will receive normal rainfall this northeast monsoon," he said.

The year 2019 can easily be classified as a year of extremes. Early this year, more than half the country was facing acute drought. There was a delay in the arrival of the southwest monsoon. But, when it did, it arrived with a bang. Several states were inundated leading to huge crop and infrastructure losses and loss of lives. Some parts of the country, such as Sangli and Kolhapur in Maharashtra faced unprecedented floods.

"We need to embrace the fact that extremes are the new normal now and it's going to be like this, or possibly become worse in the future," warned Balasubramanian. According to him, how tropical waves respond to a changing climate is not very clear and needs further research. But, it can convincingly be said that due to the changing climate and warming of the oceans, all the rain-bearing systems have a tendency to get intensified beyond their normal strength, resulting in extreme rainfall events.

A recent press release of IMD notes this year there were 11 cyclonic disturbances over the north Indian Ocean of which four were over the Bay of Bengal and seven over the Arabian Sea. These include three cyclones in the Bay and four in the Arabian Sea.

"Thus, the Arabian Sea has become more active during 2019 with the formation of 7 CDs [cyclonic disturbances] against the normal 1.7 CDs per year. Similarly, 4 cyclones have developed over the Arabian Sea against the normal of 1 per year," reads the December 4 press release.

It goes on to note that "considering the past data (1981-2018), the year 2019 witnessed the formation of the maximum number of CDs so far over the Arabian Sea… The year 2019 also witnessed the development of more intense cyclones over the Arabian Sea."

The need of the hour is to have an effective communication system between the weather departments, the state governments and the civic authorities. Also, states, cities and villages need to work towards climate resilience.

In 2013, Tamil Nadu prepared its draft Tamil Nadu State Action Plan on Climate Change, which was endorsed by the Union ministry of environment, forest and climate change in March 2015. It noted the "annual rainfall is predicted to increase considerably towards the end of the century (2081-2100) in Tamil Nadu. It further read "rainfall intensity during the north east monsoon is likely to increase by 9-22 mm/day by the end of the century across the state, with heavier precipitation towards the coast".

The state action plan was to be revised by March 31 this year taking into consideration India's intended nationally determined contributions (INDCs) to the United Nations Framework Convention on Climate Change. It was to be submitted to the Union environment ministry in September this year. However, as per the news reports, the revised plan is still not final.